Patent GB-652059-A (GB)
Title: Improvements relating to the generation of picture signals for television transmission
Inventor: JOHNSON CHARLES ALAN
IPC: H04N3/40
Filed: 1948-06-03
Abstract: 652,059. Cathode-ray tubes. ELECTRIC & MUSICAL INDUSTRIES, Ltd. June 3, 1948, No. 14961. [Class 39 (i)] [Also in Group XL (b)] In a cathode-ray tube for generating picture signals for television transmission of the kind having a target capable of storing charges and on which an image is projected during intervals, the intensity of the image and also the potential of highly-positive electrodes such as the wall anode, ion trap, and decelerating electrode are reduced during these intervals in order to reduce saturation of the target electrode and consequent instability. A television transmission tube 1, Fig. 1, has a wall anode 3 and a target comprising a transparent signal plate 4 and a mosaic of conductive photo-sensitive elements 6 insulated from the plate 4 by a transparent insulating layer. Annular electrodes 7, 8 serve as an ion trap and a decelerating electrode. The tube 1 is enclosed in a solenoid 9 which produces an axial focusing field. A load impedance 11 is connected to the signal plate and voltages set up across it are applied to a vision amplifier 12. An image is projected on the mosaic 6 from a kinematograph film 16 by means of a light source 17 and optical system 18, 20. A master oscillator 23 generating a 50-cycle pulse waveform controls the generation of line and frame frequency saw-tooth scanning waveforms and also controls a suppression pulse generator 24 and a black-out pulse generator 25. The suppression pulses commence a short time before the commencement of the return stroke in the frame frequency scanning waveform and terminate a short time after termination of the return stroke. They are applied to the amplifier 12 with such amplitude and phase as to suppress the output thereof during each suppression pulse. The black-out pulse generator 25 generates frame frequency pulses occurring wholly within the time duration of the suppression pulses but overlapping the return strokes of the frame scanning waveform. These pulses are applied to the modulator electrode of the gun 2 to extinguish the beam during the return strokes and also to a switching pulse generator 26 which generates pulses wholly within the time duration of the black-out pulses. The switching pulses are applied to three switching circuits 27, 28, 29 which control the potential applied to the wall anode 3, ion trap 7, and decelerating electrode 8. Between occurrences of the switching pulses, when scanning of the mosaic screen 6 by the beam of electrons from the gun 2 occurs, the circuits 27, 28, 29 apply to the electrodes 3, 7, 8 potentials of, say, 200, 300 and 50 volts with respect to the cathode of the gun 2, shown as earthed. The anode of the gun 2 is connected to a source of potential of about 200 volts, the load impedance 11 being earthed. During occurrence of the switching pulses the potentials applied to the electrodes 3, 7, 8 are switched to about 5 volts positive in each case. A shutter 30 is driven by a synchronous motor 31 so that it rotates once per film frame and has an aperture so dimensioned that the light image is wholly obscured during the entire intervals between successive switching pulses, but during the occurrence of these pulses the light image is projected on the mosaic 6 and causes the elements thereof to acquire positive potentials representative of the elements of the image. Fig. 2 shows the switching circuit 27, the circuits 28, 29 being similar. It comprises three diodes 32, 33, 34 the anodes of the diodes 32, 33 being connected through a condenser 35 to the switching pulse generator 26. The cathodes of the diodes 33, 34 are connected to the wall anode 3 and also through a high resistance 38 to a potential source 39 of the order of 300 volts negative. The anodes of the diodes 32, 33 are connected to a source of positive potential 36 of the order of 300 volts by a resistance 37, whilst the cathode of the diode 32 is connected to the tapping of a potentiometer 40 and to earth by a smoothing condenser 41, the potentiometer 40 being connected in series with a resistance 42 between the positive source 36 and earth. The anode of the diode 34 is connected to the tapping of a potentiometer 43 and to earth by a smoothing condenser 44, the potentiometer 43 being connected in series with a resistance 45 between the potential source 36 and earth. The ratio of the resistance of 40 to 42 is much greater than the ratio of the resistance of 43 to 45. The tapping on 40 determines the higher potential applied to the wall anode 3 whilst the tapping on 43 determines the lower potential applied to the same electrode. The switching pulses from 26 are applied in negative sense to the anodes of the diodes 32, 33 and in the intervals between their occurrences the diodes 32, 33 conduct and the potential of the tapping on 40 is applied to the wall anode 3. The amplitude of the pulses applied to the anodes of the diodes 32, 33 is greater than the difference between the potentials at the tappings on the potentiometers 40, 43 so that during the occurrence of each pulse the diodes 32, 33 are rendered non-conducting, the diode 34 then becoming conducting so that the wall anode potential falls to that of the tapping on 43. The mosaic is illuminated only during intervals when these lower potentials are applied to the electrodes 3, 7, 8, which cannot then saturate the photo-electron emission from elements of the mosaic which have already become more than a predetermined number of volts positive. The application of suppression pulses to the vision amplifier 12 prevents undesirable signals set up across the impedance 11 due to the switching of the potentials on the electrodes 3, 7, 8 from appearing in the output of the amplifier 12. A camera type shutter may be employed instead of the disc shutter 30. Instead of a light image being projected on to the mosaic 6 it may be projected on to a photocathode and the electron image therefrom projected on to the target. The shutter 30 may then be replaced by mechanism for switching the electron image off and on. The ion trap electrode 7 may be a mesh transverse to the axis of the tube and connected within the tube to the wall anode 3, the switching circuit 28 then not being required. Specifications 449,242 and 652,060, [Group XL (b)], are referred to.